1. Field of the Invention
The present invention relates to a magnetic recording medium. More particularly, the invention relates to a magnetic recording medium in the form of tape such as an audio-tape or a video-tape which shows excellent running property even after the medium is exposed to a high temperature.
2. Description of Prior Arts
A magnetic recording medium in the form of tape (e.g., an audio-tape or a video-tape encased in a cassette) has been conventionally employed or stored in an environment having less temperature variation such as in an interior of house. However, a radio cassette tape-recorder (i.e., a cassette tape-recorder with radio), a stereo record player for automobiles or a small-sized video tape recorder, etc. are widely employed in recent years, the magnetic recording media are now frequently employed or stored in a severe environment such as an environment of high temperature, environment of high temperature and high humidity, or environment of low temperature. For instance, it is reported that the highest temperature in an automobile interior is 104.degree. C. during day time in the summer season according to the high-temperature or low-temperature exposure test of automobile parts (JIS D0204).
A magnetic recording medium in the form of tape exposed to such a high temperature is apt to shrink to curl, or to shrink more greatly on the edge of one side than that of the other side to have a bow shape in the longitudinal direction. Otherwise, a hub of a cassette half is likely deformed or ejected out, because the magnetic recording medium shrinks to excessively tighten the hub. Further, when a magnetic recording medium such as an audio-tape recorded with music is exposed to high temperature, not only the output level greatly varies in a reproduction process to cause abnormal reproduction, but also the magnetic recording medium shows poor running property. Especially in the case of using an audio-tape having a thin support (e.g., audio compact cassette tapes such as C-80 type, C-90 type and C-120 type), the above-mentioned troubles caused by the shrinkage of tapes markedly take place, because the tape encased in a casette half is so long that the winding times of tape around the hub increases.
In the case of a video cassette tape, the recent wide use of portable VTRs request the tape to have much more improved running property and durability than the conventional ones. While tapes need to be highly durable, the tape thickness tends to be thinner, for example, less than 20 .mu.m. A video-tape exposed to a high temperature is likely to produce tape distortion caused by the heat shrinkage as well as in the above-mentioned audiotape. Further, other characteristic problems of video tapes such as an occurrence of skew in a reproduction process tend to be brought about.
For coping with the above-mentioned troubles probably caused by the heat shrinkage of tapes which occurs after storage at a high temperature, there has been proposed a method of adjusting a draw ratio of a plastic sheet (base) for the preparation of a support so as to produce a support having a low heat shrinkage ratio. The troubles caused by the heat shrinkage of tapes can be improved to a certain level by such method, but the level is still unsatisfactory.